Next time you are waiting for your turn at the dentist's chair, why not let your mind wonder about something more appealing? Here’s a relevant topic: A long time ago tooth decay was a rare occurrence. Our early relatives, including the Neandertals, hardly ever had caries. In fact, we may owe our regular trips to the dentist not to unhealthy eating habits but to recent changes in the bacteria inhabiting our mouths—and maybe even to rats!

You could be forgiven for thinking that tooth decay is an inevitable fact of life; even ancient Egyptians practiced dentistry. But the study of human teeth suggests that before our ancestors started cultivating plants for food, cavities were uncommon. Tooth decay, it seems, spread once we changed to an agricultural lifestyle.

New evidence from Omar Eduardo Cornejo Ordaz, a postdoctoral research fellow at the Stanford School of Medicine, and his colleagues back up this hypothesis. They analyzed the genomes of several strains of the prevalent caries-causing bacterium, Streptococcus mutans, to determine when new genes evolved in this species and its close relatives. The team's statistical analyses suggest the bacteria's population started expanding exponentially about 10,000 years ago, which coincides quite nicely with the birth of agriculture.

But how did agriculture spark a tooth decay revolution? Cornejo Ordaz thinks that even though agriculture brought us improved living conditions, it also brought humans and rats into much closer proximity than before. According to Cornejo Ordaz, the species most closely related to S. mutans is Streptococcus ratti, and its “natural environment is probably the rat's mouth. It is easy to imagine,” he says, “that when population densities increased after the origin of agriculture, there was increased possibility for a host shift and development of a new species."

Other researchers agree that a switch to agriculture proved damaging to the dental health of several societies. George Armelagos, an anthropologist at Emory University, studied the teeth from 39 fossil skeletons found in Wadi Halfa, Sudan, estimated to be between 8,000 to 11,000 years old. Their findings, published online December 10 in Molecular Biology and Evolution, suggest that when the Nubian people living in northern Sudan and southern Egypt switched to intensive agriculture, the incidence of caries in the population jumped from 0.8 percent to nearly 20 percent.

Not all researchers agree with the rat hypothesis, however. Peter Brown, a paleoanthropologist from the University of New England in Australia, thinks the emergence of tooth decay relates to sugar in the human diet. “You need to think about the distribution of rats and caries, both in terms of geography and through time,” Brown says. “In Australia caries came with the introduction of sugar and flour to Aboriginal communities. In Japan, during the Edo period, the samurai class had relatively poor oral health (sugar and refined carbohydrates) but the common people had a very low caries incidence (no access to sugar or refined carbohydrates)—nothing to do with rats.”